This invention relates to an H-plane electronically scanned yagi antenna system. More particularly, the invention relates to an antenna system which includes parasitic reflecting rods to broaden the H-plane element pattern to provide electronic scanning over a ninety degree (90.degree.) angular sector. The proposed system is suitable for use in aircraft installations where vertical space is limited such as in the edges of an airfoil. One specific application is for Information Friend or Foe (I.F.F.) antenna systems, where a vertically polarized beam is required.
In general, the use of multiple antenna elements in an array provides improved directivity and antenna gain in a system adaptable to electronic scanning wherein the beam axis is scanned by controlling the phase of the radio signals used to excite the individual antenna elements.
Antenna arrays, however, present problems absent in non-arrayed systems. Preferably, the individual elements of the array should be as closely spaced as possible, consistent with the desired gain and directivity of the beam, to maintain the compactness of the system and prevent grating lobes. This close spacing, however, makes it difficult to avoid inter-element "feed through" where radiation from one driver tends to be received by neighboring drivers. An electrical current is thereby induced in the neighboring channel through its respective antenna port. This current tends to introduce irregularities into the channels's feed line and signal processing system. Consequently, this inter-element coupling has a substantial impact on the radiation pattern and results in an overall degradation of the system response.
As mutual coupling increases, inter-element isolation is reduced and the antenna becomes less effective due to the element pattern mismatch and resulting reflections emanating therefrom. Non-uniformities in the array field develop which induce variations in element input impedance as the beam point direction changes. When transmitting, this impedance variation produces a mismatch between the antenna impedance and the signal generator impedance, thereby reducing maximum power transfer. On receive, the apparent impedance mismatch results in received signals being partially reflected back into space by the antenna. In either case, both antenna pattern and power gain (or efficiency) are adversely affected.
In order that the antenna array present a uniform impedance and exhibit radiation uniformity, it is desirable (if not a prerequisite) that the in-array element pattern have a smooth contour and that its relative gain be substantially constant over the scanning angle to be serviced. This radiation uniformity can be obtained, and is obtained in the present invention, by substantial elimination of energy loss due to inter-element feed through, thus allowing this energy to be reradiated as is desired.
In the past, reduction or elimination of inter-element coupling has been sought by the placement of metal septa between elements to block or inhibit energy transfer between such elements. These septa form metal planes extending from the reflector (in the yagi array) to the forward director. The use of such metallic septa between elements is not an acceptable solution to the problem for several reasons. The major limitation to this technique is that it restricts the angular sector over which the array may be scanned due to the effects of both diffraction and mode elimination.
While the metallic septa are useful in reducing mutual coupling between adjacent drivers, they also act as waveguides which limit the modes of propagation excitable by the antenna. As modes of propagation become suppressed, nonuniformities in the system response develop. This limits the angular sector through which the system can effectively operate thereby rendering this construction unacceptable for wide-angle scanning applications.
Another undesirable consequence of the use of metallic septa is the introduction of diffraction effects into the field pattern. As with the limitation of modes of propagation, the result is a degradation of the field pattern and a corresponding reduction in angular scanning capacity of the system.
The use of various other techniques to increase inter-element isolation such as balancing the feed line have also been proposed as a means for stabilizing the element impedance over a wide-angle of operation. These techniques compensate for the effects of inter-element coupling by electronic processing of the signal at a location some distance from the antenna. This procedure produces satisfactory results in some applications, yet the space and weight requirements of the balancing device make it inappropriate for use in numerous situations.
The present invention, however, calls for an H-plane array wherein the elements are parallel to each other rather than adjacent. This antenna construction results in a mutual coupling field broadside to the elements rather than at the element ends. Many previous antenna construction techniques concerned with coupling between adjacent elements are thus ineffective in an H-plane array.
Techniques involving the use of parasitic reflectors in scanning systems have been proposed in the U.S. Pat. No. 2,409,944 issued to Loughren and U.S. Pat. No. 2,629,865 issued to Barker. In both cases, involving E-plane arrays, the parasitic elements were used as reflectors to increase the directivity of the dipoles in desired direction of radiation. In so doing, the depth of each element is increased while the problem of maximizing array element isolation is not addressed.
Accordingly, principle objects of the invention are to provide increased antenna coverage and improved power transfer characteristics in antenna array systems that are electronically scanned in the H-plane.
Other objects of the invention include providing an antenna array system having low mutual electromagnetic coupling between individual antenna elements and having smooth in array element patterns and relatively high and uniform gain over a finite bandwidth.